The present invention relates to a method of optimising the standard weight deviation of cigarettes on a dual-rod cigarette manufacturing machine.
On modern single-rod cigarette manufacturing machines, cigarettes are formed starting with a batch of shredded tobacco carded inside a supply unit having an outlet in the form of a vertical duct.
In actual use, an upward stream of tobacco particles flows up the said duct and on to the underside of a suction type conveyor belt, so as to form a continuous layer or bead of tobacco, which is fed by the suction conveyor through a shaving station for evening off the thickness of the bead.
The bead is then fed into what is known as a filling station where it is gradually loaded by the suction conveyor on to a continuous strip of paper. The said paper strip and he continuous bead of tobacco placed on top of it are then fed through a forming beam, along which the opposite side edges of the strip are folded together about the tobacco bead and stuck together to produce a continuous cigarette rod, which is then cut into given cigarette lengths by a rotary cutting head.
One of the major requirements of cigarette manufacturing machines of the aforementioned type is that the weight of the cigarettes should be ad consistent as possible. For this purpose, cigarette manufacturing machines of the aforementioned type are known to include, not only a weighing device for rejecting cigarettes not conforming in weight with a given acceptance range, but also shavers connected to the said weighing device, for continuously and dynamically correcting the weight of the cigarettes, and ensuring it conforms as closely as possible with a given value within the said acceptance range.
In particular, the said shavers are known to be connected to a weighing and control unit designed to weigh each cigarette as it comes off the machine; to display the results, relative to groups of a given number of cigarettes, in the form of a Gaussian curve, usually showing the cigarette weight and number on Cartisian coordinate Y and X axes respectively; and to supply signals indicating the maximum, minimum and mean weight, and standard deviation or variance of the curve.
The standard variation is, of course, reflected in the size of the curve, and is an indication of the quality level being attained. That is to say, the tighter the curve, i.e. the lower the standard deviation of each curve, the more consistent is the cigarette weight within each sample group.
Maintaining the standard deviation below a given value within the acceptance range, however, is not the only concern of cigarette manufacturers. In addition to a good standard of quality, machines are also expected to provide for economy, which means maintaining the minimum weights on the curves as close as possible to the lower acceptance range limit.
For this purpose, the output signals from the said weighing and control unit are usually supplied to a processor for obtaining an error signal as a function of the difference between the minimum weight on the curve and the lower acceptance range limit, the said shavers being connected to the weighing device via the said processor, for maintaining the said error substantially equal to zero.
A dual-rod cigarette manufacturing machine is described, for example, in U.S. Pat. No. 4,336,812.
On machines of this sort, a single stream of tobacco particles is fed into an up-duct, in turn, supplying two parallel up-ducts, each terminating underneath a respective suction type conveyor belt. The beads formed on the belts are shaved by shavers and deposited on to continuous paper strips, which are wrapped about the beads to produce continuous cigarette rods, which are then cut into cigarette lengths by a single rotary cutting head.
On dual-rod cigarette manufacturing machines of the aforementioned type, cigarette weight may be controlled by connecting the shavers to a weighing and control unit, as on single-rod type machines.
The performance of such a system may be considered as being of the same standard as on a single-rod machine, if the cigarettes formed from each rod are judged separately, as though produced by two separate single-rod machines.
Quite the opposite applies, however, if the cigarettes from both rods are considered jointly, i.e. as being produced by the same dual-rod machine. In this case, the difference in weight between the cigarettes in each rod is far greater than on single-rod machines. In other words, dual-rod machines present a far higher standard weight deviations than single-rod machines.
The drawback of such a situation is that, downstream from the said cutting head, the cigarettes from both rods are processed jointly, i.e. are fed into the same filter assembly machine, and mixed together inside the finished packs.